1. Field of the Invention
The present invention relates to a liquid crystal display device using a switching device such as a thin film transistor, and in particular to a reflection-type liquid crystal display device including a reflection electrode formed of a metal material connected to a switching device and a method for producing the same. In this specification, a liquid crystal display device will be referred to as an "LCD device", and a thin film transistor will referred to as a "TFT".
2. Description of the Related Art
Recently, semiconductor devices such as ICs and LSIs, and industrial- and home-use electric and electronic devices and appliances including such semiconductor devices have been developed and sold in a great quantity in the market. VTRs and personal computers in addition to TV receivers are now in wide use by the general public and have become commonplace. Among these devices and appliances, LCD devices are a target of attention as a type of display device having advantages of lightness and low power consumption. Active matrix LCD devices including TFTs respectively connected to a plurality of pixels for controlling the pixels receive special attention as providing superior resolution and clear images.
One type of conventional active device is a TFT including an amorphous silicon thin film. Many types of active matrix LCD devices including such a type of TFTs have been developed into products. Today, as an active device for driving a pixel electrode, a TFT including a polycrystalline silicon thin film, which can be formed on the same substrate with a circuit for driving the TFT, is expected to be a positive replacement for the amorphous silicon TFT. A technology for forming such a polycrystalline silicon TFT is now being actively studied.
A polycrystalline silicon thin film has a higher mobility than an amorphous silicon used in the conventional TFTs and thus provides higher performance when used in a TFT. TFTs for driving a pixel electrode which are formed on the same substrate integrally with a circuit for driving the TFTs will significantly reduce the production cost.
Technologies now proposed for forming a polycrystalline silicon thin film used as an active layer of the polycrystalline silicon TFT on a glass substrate include a solid phase growth method and a laser crystallization method. According to the solid phase growth method, amorphous silicon is deposited on a glass substrate and then crystallized by heat-treatment performed at a temperature of about 600.degree. C. for several to several tens of hours. According to the laser crystallization method, amorphous silicon is melted and thus recrystallized instantaneously by irradiation with a pulse laser such as excimer laser.
A plurality of pixel electrodes are respectively connected to drains of TFTs and are spaced apart by a prescribed distance from gate lines and source lines adjacent to the pixel electrodes. As a structure of the pixel electrodes and elements disposed in the vicinity thereof, a pixel-on-passivation structure shown in FIG. 15 has been proposed. The pixel-on-passivation structure includes a substrate 51 including the TFT, an interlayer insulating layer 58 formed of a polyimide resin or an acrylic resin, and a pixel electrode 64 provided on the interlayer insulating layer 58. A drain electrode 61 of the TFT is connected to the pixel electrode 64 through a contact hole 63 formed in the interlayer insulating layer 58. Reference numeral 62 represents a source electrode of the TFT.
According to such a structure, the pixel electrode 64 is insulated from the gate lines and source lines by the interlayer insulating layer 58 formed of a polyimide resin or an acrylic resin. This arrangement allows for the pixel electrode 64 to be provided so that ends of the pixel electrode 64 overlap the gate lines and source lines. Thus, the effective area of the pixel electrode 64, i.e., the numerical aperture, is increased. Moreover, the interlayer insulating layer 58 compensates for the stepped surface caused by the TFTs, gate lines and source lines and provides a flat surface. Such a flat surface has an effect of minimizing the disturbance of alignment of liquid crystal molecules 60.
Active matrix LCD devices having such a structure are mainly classified into a transmission-type LCD devices in which the pixel electrodes are formed of ITO (indium tin oxide) and reflection-type LCD devices in which the pixel electrodes are formed of a reflective material such as a metal material. Since the LCD devices do not include a light source therein, the transmission-type LCD devices use a lighting device, i.e., a so-called backlight disposed behind the LCD devices to perform display. Reflection-type LCD devices perform display by external light which is reflected by the reflection electrodes.
Transmission-type LCD devices require a high power consumption due to the use of backlight but have an advantage of realizing bright and high-contrast display regardless of the brightness of the surroundings. Reflection-type LCD devices have a drawback of the brightness and the contrast of the display being influenced by the brightness of the surroundings and environment in which they are used but have an advantage of a very low power consumption. Therefore, the reflection-type LCD devices attract attention as display devices usable for mobile information devices using a battery as a power source.
In conventional LCD devices, the reflection electrodes are generally formed of aluminum or an alloy thereof since these materials have relatively high reflectance, are easy to form into a film by sputtering or the like, and allow for high etching precision.
When aluminum is used for a mirror-surface reflection electrode having directivity with respect to an angle of incident light or a scattering surface reflection electrode having little directivity, the light utilization factor caused by reflection needs to be considered.
FIG. 13 is a graph illustrating the absolute reflectance of silver and aluminum layers and the thickness of the silver and aluminum layers. The reflectance is measured where light is incident on an Ag sample vapor-deposited at room temperature by resistance heating from above at a substantially vertical angle (angle of incidence: 12 degrees).
As shown in FIG. 13, the reflectance of aluminum is about 90% in air and about 85% in a liquid crystal layer. As can be appreciated from these results, use of aluminum for a mirror-surface reflection electrode or a scattering surface reflection electrode has a problem in that the intensity of the incident light is reduced by 10 to 15% and that the reduced intensity of light is absorbed by aluminum to cause heat generation.
In order to improve the reflectance of aluminum, a dielectric multi-layer film can be formed on the surface of the aluminum film. Such an electrode is difficult to produce due to the high precision required for the dielectric film and the need for forming a plurality of dielectric layers, thus raising the production cost. The dielectric multi-layer film is formed of an insulating material, which requires a higher voltage for driving liquid crystal molecules compared to the case in which the dielectric multi-layer film is not used.
Japanese Laid-Open Publication Nos. 56-57086 and 57-120977, for example, propose use of silver which has a higher reflectance than aluminum, for a reflection electrode. As shown in FIG. 13, the reflectance of silver is about 5% higher than that of aluminum and thus is suitable to be used for a mirror-surface reflection electrode or a scattering surface reflection electrode.
Whereas aluminum allows for precision processing for patterning of about 2 .mu.m or less by anisotropic etching using chlorine plasma, silver does not allow for anisotropic dry etching using halogen gas because the vapor pressure of halogenated silver of AgCl (silver chloride), AgF (silver fluoride) and the like is excessively low. Accordingly, the precision processing technology by dry etching of silver has not been established. Silver is mostly patterned by wet etching using a nitric acid etchant. With wet etching, i.e., isotropic etching, it is difficult to realize processing as precise as is possible with aluminum.
In formation of LCD devices, precision processing is very important for forming signal lines and reflection electrodes. As the area of one pixel area becomes smaller along with reduced size and higher precision of the LCD devices, the width of the signal lines and the size of inter-reflection electrode spaces have a significant effect on the numerical aperture of the LCD devices. Use of silver for the reflection electrodes, which requires a larger size of inter-reflection electrode spaces in order to compensate for size shifting due to over-wet etching, has a problem in that the numerical aperture is reduced.